1. Field of the Invention
The present invention relates to a system and a method for detecting deterioration of an oxygen sensor, and more particularly, to a system and a method for detecting deterioration of an oxygen sensor used in a feedback type air-fuel ratio control system of an internal combustion engine.
2. Description of the Prior Art
Japanese Patent First Provisional Publications Nos. 58-47248 and 59-215935 show conventional feedback type air-fuel ratio control systems for an internal combustion engine. In these systems, a basic fuel injection quantity is calculated based on various engine operation informations, such as, intake air amount, engine speed and the like, and the basic fuel injection quantity is corrected in accordance with an information signal issued from an oxygen sensor installed in an exhaust gas conduit. The amount of fuel practically fed to the engine is controlled in accordance with the corrected fuel injection quantity. This control is repeated in a feedback manner for keeping the air-fuel ratio of air-fuel mixture within a desirable or stoichiometric level.
FIG. 16 shows a block diagram of the conventional air-fuel ratio control system of feedback type.
Designated by reference "A" is a computer-installed control unit. A basic pulse is produced from signals respectively issued from an air-flow meter and an ignition coil. The basic pulse is weighted by an information signal from a throttle valve switch and corrected in voltage by a battery. Furthermore, the pulse is weighted, through an air-fuel ratio feedback control circuit "B", by an information signal issued from an oxygen sensor 1 and weighted by information signals respectively issued from a start switch and a coolant temperature sensor. The corrected signal is then treated by an arithmetic circuit and amplified by a power amplifier to practically actuate fuel injectors.
FIG. 17 shows the detail of the air-fuel ratio feedback control circuit "B" shown in FIG. 16. When the electromotive force of the oxygen sensor 1 is higher than a reference voltage, it is judged that the air-fuel mixture practically fed to the engine is richer than stoichiometric. Upon this, the circuit "B" adds a so-called "mixture leaning signal" to the basic signal for controlling the fuel injectors to inject smaller amount of fuel. While, when the electromotive force is lower than the reference voltage, it is judged that the air-fuel mixture fed to the engine is leaner than stoichiometric. Upon this, the circuit "B" adds a so-called "mixture enrichment signal" to the basic signal for controlling the fuel injectors to inject larger amount of fuel. In fact, the amount of fuel injected by the fuel injectors is controlled by varying the time during which the injectors are opened.
FIG. 18 is a chart showing an output signal V' of the oxygen sensor 1 on the axis of ordinates and elapsed time on the axis of abscissas. FIG. 19 is a graph showing an air-fuel ratio correction factor (.alpha.) on the axis of the ordinates and elapsed time on the axis of abscissas. It is to be noted that proportional factors PR and PL and integral factors IR and IL shown in FIG. 19, which are the correction factors, are all constant.
Under the above-mentioned control, a catalytic converter (particularly, three-way type catalytic converter) can exhibit high performance in purifying the exhaust gas from the engine. However, hitherto, the above-mentioned conventional air-fuel ratio control system has been constructed without taking a severe consideration on deterioration of the oxygen sensor which appears with passing of time. In fact, the output characteristic of the oxygen sensor changes with the lapse of time. Thus, after prolonged usage of the system, the stoichiometrically controlled feeding of air-fuel mixture to the engine becomes out of order due to the deterioration of the oxygen sensor and thus the exhaust gas from the engine fails to have an exhaust composition which is suitable for allowing the catalytic converter to exhibit its maximum performance.